Mei Health Dictionary

Meiosis, or reduction division, is the form of cell division that only occurs in the gonads (see GONAD) – that is, the testis (see TESTICLE) and the ovary (see OVARIES) – giving rise to the germ cells (gametes) of the sperms (see SPERMATOZOON) and the ova (see OVUM).

Two types of sperm cells are produced: one contains 22 autosomes and a Y sex chromosome (see SEX CHROMOSOMES); the other, 22 autosomes and an X sex chromosome. All the ova, however, produced by normal meiosis have 22 autosomes and an X sex chromosome.

Two divisions of the NUCLEUS occur (see also CELLS) and only one division of the chromosomes, so that the number of chromosomes in the ova and sperms is half that of the somatic cells. Each chromosome pair divides so that the gametes receive only one member of each pair. The number of chromosomes is restored to full complement at fertilisation so that the zygote has a complete set, each chromosome from the nucleus of the sperm pairing up with its corresponding partner from the ovum.

The ?rst stage of meiosis involves the pairing of homologous chromosomes which join together and synapse lengthwise. The chromosomes then become doubled by splitting along their length and the chromatids so formed are held together by centromeres. As the homologous chromosomes – one of which has come from the mother, and the other from the father – are lying together, genetic interchange can take place between the chromatids and in this way new combinations of GENES arise. All four chromatids are closely interwoven and recombination may take place between any maternal or any paternal chromatids. This process is known as crossing over or recombination. After this period of interchange, homologous chromosomes move apart, one to each pole of the nucleus. The cell then divides and the nucleus of each new cell now contains 23 and not 46 chromosomes. The second meiotic division then occurs, the centromeres divide and the chromatids move apart to opposite poles of the nucleus so there are still 23 chromosomes in each of the daughter nuclei so formed. The cell divides again so that there are four gametes, each containing a half number (haploid) set of chromosomes. However, owing to the recombination or crossing over, the genetic material is not identical with either parent or with other spermatozoa.... meiosis

Numerous glands within the tarsal plates of the eyelids. Their secretions form part of the tears. (See EYE.)... meibomian glands

See chalazion.... meibomian cyst

(Chinese) One who is virtuous and beautiful

Meishana, Meishawn, Meishaun, Meishon... meishan

(Chinese) One who is forever enchanting... meiwei

Beat. Privatdocent ETH, Dr sc nat Pharmacist. Head of R & D Dept at Zeller AG, Herbal Medicines, Romanshorn, Switzerland. Medical plant researcher at ETH, Department of Pharmacy, Zurich. Developed modern strategies for analysis of herbal medicines, especially in the field of HPLC. University teacher in pharmacognosy and phytochemistry. C/O Zeller AG, Ch-8590 Romanshorn, Switzerland. ... meier

A rare condition in which a tumour of an ovary is accompanied by ascites and a pleural effusion. The fluid usually disappears when the tumour is removed.... meig’s syndrome

n. inflammation of the *meibomian glands of the eyelids.... meibomianitis

the rare combination of a benign ovarian *fibroma with *ascites and a right-sided pleural effusion. [J. V. Meigs (1892–1963), US gynaecologist]... meigs syndrome

(submucous plexus) a fine network of parasympathetic nerve fibres in the wall of the alimentary canal, supplying the muscles and mucous membrane. [G. Meissner (1829–1905), German physiologist]... meissner’s plexus